As one common medical instrument, an automatic injection device can accurately expel a desired dosage without manual needle insertion, injection and the like.
Patent No. WO2004/078239A1 has disclosed a drug delivery device. The drug delivery device includes: a housing having an internal thread, a dosage dial sleeve having a thread engaged with the internal thread of the housing, a rotary sleeve releasably connected to the dose dial sleeve, and a clutch means located between the dose dial sleeve and the rotary sleeve, wherein, when the dose dial sleeve and the rotary sleeve are coupled, both the sleeves are allowed to rotate with respect to the housing. When the dose dial sleeve and the rotary sleeve are de-coupled, rotation of the dose dial sleeve with respect to the housing is allowed, whilst rotation of the rotary sleeve with respect to the housing is not allowed, whereby axial movement of the rotary sleeve is allowed so that a force is transferred in the longitudinal direction to the proximal end of the drug delivery device. In this drug delivery device, there are many structural parts, and the components have highly required machining precision and great assembly difficulty. For example, a thread matched with a first thread of the piston rod is provided on an inner wall of the rotary sleeve, and a second thread is provided on the piston rod; the second thread and the first thread are arranged reversely; a flexible and helically extending arm is provided on a clicker means in transmission with the clutch means; a toothed member dis provided on the arm in a particular direction; and the like. Moreover, when in use, the internal components easily get stuck or are easily worn out, so that it is likely to result in the failure of the injection device.
Patent No. CN101184521A has disclosed a dosing device. The dosing device is arranged on an injection device or set as a part of the injection device. The dosing device has an adjustment sleeve which is provided at a rear end of the injection device and able to rotate about a longitudinal axis of the injection device, and a rotary sleeve. The rotary sleeve has an external thread which is engaged in the injection device or an internal thread of a housing of the injection device. The rotary sleeve has an internal thread into which an external thread of a locking element is engaged, so that the rotary sleeve is in threaded engagement with the injection device or the housing thereof and in threaded engagement with the locking element. The locking element is borne on a threaded rod in such a manner that it can move in the axial direction but incapable of rotating, and has two opposite guide elements to be engaged into a guide groove extending along the threaded rod in the axial direction, so that the rotation of the threaded rod is always transferred onto the locking element. The locking element rotates with the threaded rod, or in other words, can block rotation of the threaded rod when it is fully screwed into the rotary sleeve, for example. In this injection device, there are many structural parts, and the components have highly required machining precision and great assembly difficulty. For example, ratchet teeth distributed spirally need to be provided on the threaded rod matched with the locking element; threads need to be provided inside and outside of the rotary sleeve; and the like.
Therefore, the injection devices in the prior art are complicated in structure, high in machining difficulty, complicated in assembly, inconvenient in operation and high in manufacturing cost.